To multiplex a plurality of signals, for example, parallel/serial conversion or an apparatus called a serializer is used. The number of transmission lines necessary to transmit a serial signal can be one regardless of the total number of signals to be exchanged simultaneously.
For example, there is known a technique of generating a serial signal by multiplexing a video signal and a control signal using a clock signal synchronized with the video signal. According to this technique, it is possible to transmit the video signal and the control signal by one transmission line independently of the bit widths of these signals. However, according to this technique, multiplexing processing is controlled using the clock signal synchronized with the video signal. Thus, in a state in which, for example, the control signal is supplied but the video signal is not supplied, the clock signal may not be supplied either, and control signal transmission may be impossible.